1. Field of the Invention
The present invention relates generally to interferometric fiber optic acoustic sensors and, more particularly to planar and linear fiber optic sensors embedded in a low bulk modulus elastomer with either balanced or unbalanced reference fibers.
2. Description of the Related Art
The design of acousto-optic sensors is based upon the phase shift in light passing through a fiber when subjected to acoustic waves. Generally an optical fiber is attached to an interferometer sensor which consists of an acoustically sensitive sensor fiber and an acoustically insensitive reference fiber. A laser beam transmitted along the optical fiber is split by an optical coupler and transmitted through both the sensor and reference fibers. The difference in phase shift between the sensor and reference fibers is related to the strength of the sensed acoustic field. These types of sensors are described in further detail in various patents, such as U.S. Pat. Nos. 4,162,397, 4297,887, 4,363,114 and 4,994,668.
Because the phase shift which occurs within any one length of fiber immersed in a liquid is small, the fibers are arranged in the planar design in either a coil or a spiral configuration. In a linear design, acoustic sensitivity is maximized when the sensor fiber is configured as a coil wound about a polymer mandrel such as teflon or nylon. In such an arrangement, the transmission mechanism is indirect. The acoustic field generates strains within the mandrel which changes its diameter and thus the fiber length, which, in turn, modulates the phase.
The planar sensor generally is a large area flexible acoustic sensor that is easily driven into flexible modes of vibration and generally exhibits large transduction effects in bending. In the planar design, the measurement of the phase modulation is accomplished by embedding a concentrically wound or snaked sensor fiber and a concentrically wound reference fiber of an acousto-optic interferometric sensor in a thick layer of a material having a low bulk modulus. In the current technology, as set forth in U.S. Pat. No. 4,994,668, it has been found that even though the sensitivity of these sensors is high, it is not flat over a wide frequency range owing to acoustically induced acceleration effects and has a tendency to deteriorate over time due to water attack on the coating/coating and coating/glass bonding layers.